The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. The electric field is generated based on RF power signals generated by a radio frequency (RF) generator of a RF power system. The RF power signals generated by the RF generator must be precisely controlled to effectively execute plasma etching.
A RF power system may include a RF generator, a matching network and a load (e.g., a plasma chamber). The RF generator generates RF power signals, which are received at the matching network. The matching network matches an input impedance of the matching network to a characteristic impedance of a transmission line between the RF generator and the matching network. This impedance matching aids in maximizing an amount of power forwarded to the matching network (“forward power”) and minimizing an amount of power reflected back from the matching network to the RF generator (“reverse power”). Forward power may be maximized and reverse power may be minimized when the input impedance of the matching network matches the characteristic impedance of the transmission line.
Heuristic, feedback or feedforward approaches are typically used to control a RF generator to maximize power transferred to the matching network. Heuristic approaches include a set of rules that are used to direct a gradient based search method to provide a sensed response satisfying a predetermined criterion. A heuristic approach can include performing a search to tune frequency of a power amplifier circuit to minimize reverse power, increasing a step size of a search space, changing direction of the search, and initiating or ceasing a search. Heuristic approaches cannot be represented by a transfer function.
A feedback approach typically includes a feedback loop, which is used to minimize error between a power setpoint and an amount of power transferred from a RF generator and a matching network. The feedback loop may include sensors and a control module. The control module adjusts output power of an agile frequency RF power source (or power amplifier). The sensors may detect voltage, current, forward power and/or reverse power out of the power amplifier and generate sensor signals. An amount of power transferred or a difference between the forward power and the reverse power is determined. An error signal is generated based on this difference. The control module may generate a power control signal based on the error signal. The power amplifier generates RF power signals based on the power control signal from the control module. Although this approach minimizes error in power to maximize power transferred from the RF generator to the matching network, this approach is limited to adjusting power that cannot minimize non-zero reflected power.
Another feedback approach includes detecting a phase difference between sensor signals, which are generated based on a voltage and current output of the power amplifier. Frequency of the power amplifier is adjusted via a voltage-controlled oscillator in response to the detected phase difference to minimize the phase difference and/or reverse power. The phase difference based frequency adjustment approach can lead to a quantitative error, which is associated with a systematic variation in a RF power system. Systematic variations may include a phase error (or phase offset), a mismatched load, a misalignment in RF signal delivery associated with tune and load parameters of the matching network, etc. The systematic variations can prevent the phase difference from being reduced to zero and/or a reflection coefficient as plotted on a Smith chart from reaching a (0,0) point. The systematic variations can also prevent a required power transfer to a load. As a result, a calibration scheme is needed to prevent a phase offset and/or a heuristic approach is needed to minimize these systematic variations. Also, when adjusting frequency of a power amplifier based on phase, a directivity signal is needed to determine which direction to adjust the frequency to minimize the phase difference.
Yet another feedback approach detects forward power and reverse power. Frequency of a power amplifier is adjusted via a voltage-controlled oscillator based on the forward power and the reverse power to minimize the reverse power. Phase information is not utilized in this approach to minimize the reverse power.
In one feedforward approach, a feedforward loop is used to adjust capacitance of a capacitor in a matching network. Sensors are used to detect forward power and reverse power. A processor adjusts operation of a motor to change the capacitance of the capacitor based on outputs of the sensors. The processor adjusts the capacitance until the reverse power is at a minimum level.